Various communication systems are available for communicating data between two locations. For example, spread-spectrum broadband digital communication systems have long offered several advantages over narrowband communication systems, including better immunity to noise, improved multipath rejection and, in the case of signals hidden in the noise floor, a low probability of intercept. Additionally, code division multiple access (CDMA) techniques allow multiple users to be supported on the same bandwidth. Typical applications of these digital communication techniques include cellular phones and the global positioning systems. However, these existing systems are often vulnerable to spoofing, jamming, and various techniques that may intercept the communicated signals.